Radiographic imaging systems have typically made use of phosphor screens onto which x-rays, passing through a patient to be imaged, impinge. Conventionally, the phosphor screen image has been used to expose a photographic film. However, over recent years, a trend towards digital imaging has developed.
One known prior art digital radiographic imaging system is based on film image digitization. This radiographic imaging system separates the display and detection medium and allows for electronic manipulation, storage and transfer of radiographic images. However, film image digitization suffers from the same inconveniences as film handling and development and requires an additional step, making the radiographic imaging system cost and time inefficient. Furthermore, the quality of images generated by this radiographic imaging system is only as good as the original film image.
It has also been considered to couple optically a phosphor screen with an optical imaging system. Specifically, a lens is used to couple the phosphor screen to an optical imager in the form of a charge-coupled device (CCD) camera. The output of the CCD camera is fed to a processor which digitizes and displays the image captured by the CCD camera.
Unfortunately, the quality of images generated has proven to be unsatisfactory due to the fact that only a fraction of the quanta released by the phosphor screen as a result of absorbed x-rays are directed by the lens to the CCD camera. Furthermore, only a fraction of the quanta directed to the CCD camera are absorbed in the CCD camera to produce electronic charge. The signal loss resulting from this secondary quantum sink has led to a corruption in the signal-to-noise ratio (SNR) and object detectability within the optical image. Increasing x-ray exposure to deal with this signal loss is not a solution due to risk to the patient.
To overcome the coupling inefficiency associated with the above-identified system, an x-ray image intensifier has been considered and is described in an article entitled "An Amorphous Selenium Liquid Crystal Light Valve For X-Ray Imaging" published in the proceedings of the International Society For Optical Engineering (SPIE) Medical Imaging 1995 conference, volume 2432, pages 228 to 234. This x-ray image Intensifier includes a photoconductive x-ray detector for generating an optical image of an x-ray exposure. The photoconductive x-ray detector comprises a twisted nematic liquid crystal (LC) cell deposited on an amorphous selenium (a-Se) film. A CCD camera captures the optical image and feeds the captured optical image to a processor where the optical image is digitized and displayed.
In operation, a potential is applied across the photoconductive x-ray detector to create an electric field across the a-Se film. When x-rays pass through a patient and are absorbed in the a-Se film, electron-hole pairs are released within the a-Se film. The electric field in the a-Se film separates the electrons and the holes and guides the electrons and holes to opposite surfaces of the a-Se film with the electrons being guided towards the LC cell. The negative charges collected at the a-Se film and LC cell interface create potential variations across the LC cell. The potential variations across the LC cell give rise to changes in the orientation of the molecules of the liquid crystal material in the LC cell which affects the polarization state of light from an external source passing through the LC cell.
Polarizers on opposed sides of the photoconductive x-ray detector translate the changes in light polarization to changes in light transmission. The end result is that variations in the potential in areas of the a-Se film where x-rays are absorbed cause spatial variations in the intensity of light transmitted through the LC cell, thus producing an optical image of the x-ray exposure. The CCD camera captures the optical image allowing the processor to digitize and display the optical image. Although this x-ray image intensifier exhibits high resolution and low noise allowing quality optical images of x-ray exposures to be generated, improvements to such systems are continually being sought.
It is therefore an object of the present invention to provide a novel x-ray image intensifier and a method of x-ray imaging.